In order to study the allelopathy of different separation in potato onion root exudates，n-butyl alcohol，ethyl acetate，aether and chloroform were used to extract the potato onion root exudates，and the effect of the four different extractions on tomato′s seed germination and seedling growth was studied.The results showed that，the allelopathy of ethyl acetate separation component was the strongest，and it could suppress the seed germination and seedling growth of tomato.Therefore，ethyl acetate could separate the potato onion root exudates better and could be the extract phase when further study the component of potato onion root exudates.